We have been characterizing the morphologic and the hemodynamic abnormalities occurring in Newfoundland dogs with genetically transmitted subaortic stenosis. Of 176 animals studied thus far, almost one-half hemodynamic and/or morphologic evidence for left ventricular outflow tract obstruction. The obstruction is absent at birth, appearing after one month of age. The lesion presents clinically by the presence of a precordial murmur, thrill, arrhythmia, congestive heart failure, bacterial endocarditis of the aortic valve, or sudden death. Obstruction to left ventricular outflow is caused by the development of a circumferential, subaortic fibromuscular ring continuous with the anterior leaflet of the mitral valve, virtually identical to the same lesion in humans. The obstruction is associated with left ventricular hypertrophy, intramural coronary artery lesions, myocardial fibrosis, and abnormalities of myocardial blood flow.